It was established that arginine, proline, histidine, and lysine residues of proteins are among those that are oxidized to carbonyl derivatives by mixed-function oxidation systems. Arginine and proline residues are both oxidized to 5-oxo-2-aminopentanoic acid. Proline is also oxidized to pyroglutamic acid and very likely to either 3-oxo- or 4-oxo-proline. Results with a model system comprised of hydrogen peroxide, ferrous iron and iron chelating agents (the Fenton system) have shown that phenylalanine is oxidized to phenyacetaldehyde and phenylacetic acid together with ammonia and carbon dioxide. Similarly, leucine is oxidized to isovaleraldehyde and isovaleric acid. In addition, small quantities of a potent inhibitor (Ki = 1 to 10 nanomolar) of horse liver alcohol dehydrogenase is produced from both amino acids. These were isolated and shown to be the oxime derivatives of the above aldehydes. In attempts to understand the role of bicarbonate ion in the oxidation of amino acid by the Fenton system, it was found that bicarbonate stimulates the auto-oxidation of ferrous iron to ferric iron and also the reduction of ferric iron to ferrous iron by hydroxylamine.